This invention relates to processing amplifiers for three-tube color television cameras and, more particularly, to an improved form of such processing amplifier which is designed to stabilize the black level of the video signal output of an image pickup tube.
In a processing amplifier of a color television camera, corrections such as shading correction, white compression, gamma correction and black clipping are effected on the video signal output from the preamplifier circuit. Particularly, in a three-tube color television camera, any black level variations in the red, blue and green color channels have generally resulted in black images in the video output from the color encoder which are not perfectly black, but are slightly colored. This is accounted for by the facts that the characteristics of circuits elements such as diodes and transistors used in the gamma correction and black clipping circuits are variable with temperature and that, particularly in the gamma correction circuit, the variations are emphasized in the vicinity of the black level because of the gamma correction characteristics. Accordingly, circuit arrangements having substantial temperature stability have long been demanded, particularly for gamma correction and black clipping circuits, which have a large influence on the signal level in the vicinity of the black level. Measures previously taken to increase the stability of the pedestal level have been to provide the clamping circuit with appropriate temperature compensating means or, as disclosed in Paper No. 4-3 of the Proceedings of The 1976 National Convention of the Institute of Television Engineers of Japan, entitled "Development of a One-Package IC Processing Amplifier for Broadcasting Use", to arrange a feedback clamping circuit on the input side of the gamma correction circuit. Such measures, however, have not been effective fully to suppress the signal variations and, in addition, have involved an inconvenience in that a substantial amount of time is required for their adjustment.